The present invention relates to photodynamic therapy for treatment of ocular disease in general. More specifically to the use of refractive polymers in the treatment of cataracts.
Previously, the primary approach to cataract removal has focused primarily on the use of tissue ablation employing ultrasonic or laser energy. Cataracts removed by this extracapsular method require extraction of the anterior capsule and lens substance but leave intact the posterior capsule of the lens. Extracapsular surgical techniques are usually performed by technologies that rely on high-impact energy pulses to shatter the lens into fractions that must be constantly degraded further before they may be removed by aspiration. The process of lens fractionalization and removal demands a high level of skill which is tedious and time consuming.
Lens proteins and their constituent amino acids have been studied to determine their behavior in response to radiation. Although most of the amino acids of which proteins are comprised do not absorb ultraviolet light (of a wavelength greater than 220 nm) tryptophan, and to a lessor phenylalanine and tyrosine, are known to absorb significant amounts of ultraviolet light and as a result to be susceptible to structural degradation a process herein referred as xe2x80x9cphacophotolysisxe2x80x9d. Tryptophan photodegradation may, for example, be induced through irradiation at wavelengths between 240-310 nm, and the efficiency of photodegradation is influenced by a number of factors including pH, temperature and encapsulation of photodynamic components in liposomes. Liposomal preparations are particularly useful where the photosensitizing agent is a green porphyrin since it has a particular affinity for lipoproteins. Additional components including monoclonal antibodies, receptor ligands and cytotoxins can be combined with photosensitizing agents to enhance selectivity to lens tissue and efficacy of photodegradation. Although tryptophan is the least common amino acid of human proteins it is the most photolabile of amino acids and present in high concentrations in human lens tissue.
Porphyrin-type photosensitizers used in photodynamic therapy, either used singly or in conjunction with a hydrophilic polymer, can be administered into the intralenticular space to enhance degradation of lens tissue (phacophotolysis). The photosensitive agents and their various formulations are generally known in the art.
Modern lasers are capable of producing picosend (10-12 second)bursts of extremely high intensity. In a picosecond pulse the photon intensity is quite high. Photosensitive chemicals in lens tissue can be actuated by lasers of variable power and wavelength. Hence, degradation of lens tissue by phacophotolysis provides a novel and unique method of cataract removal not heretofore practiced by prior art.
A search of the prior art did not disclose any patents that read directly on the claims of the instant invention however, the following U.S. patents were considered related:
The U.S. Pat. No. 4,078,564 issued to Spina discloses an enzymatic intralenticular cataract treatment for removal of nuclear cortical and subcapsular regions of the cataractous lens through enzymatic digestion thereof The treatment comprises introduction of a concentrated solution of mixed exogenous enzymes into the nuclear and cortical regions of a cataractous lens, and after enzymatic digestion removing the liquefied cataractous material. The procedure allows removal of the nuclear, cortical and subcapsular portions of a cataractous lens through a very tiny incision in the eye and lens capsule, leaving all other structures within the eye intact.
The U.S. Pat. No. 4,608,050 issued to Wright discloses a method of treating defective lenses in the eye, wherein the natural lens is removed leaving the lens capsule intact. A curable liquid composition comprising a crosslinkable siloxane polymer, a crosslinker and a crosslinking catalyst is injected into the lens capsule where the composition cures at body temperature to a solid, transparent synthetic lens which remains clear in the presence of physiological fluids.
The U.S. Pat. No. 4,919,151 issued to Grubbs, et al discloses a synthetic polymer for endocapsular lens replacement in an eye. The polymer, which is injected into the lens capsule after removal of the lens, comprises an oxygen-stabilized photosensitive prepolymer. An example of such a prepolymer comprises polyether with urethane linkages with one or both ends capped with a functional group containing at least one double bond, such as an acrylate, a methacrylate, or a styrene. The polymerization reaction is initiated with a photoinitiator such as dimethoxyphenylacetophenone and is quenched in the presence of oxygen. Contrary to the prior art polymers, the time of curing is approximately one minute. The viscosity and thickness of the polymer formed may be tailored to achieve a desired index of refraction of between about 1.3 and 1.6.
The U.S. Pat. No. 5,022,413 issued to Spina, Jr. et al discloses a method for treating cataracts by introducing a lenticular tissue dispersing agent into the opacified lens through a small opening in the lens capsule so that the capsule remains substantially intact. The tissue dispersing agent is contained in the lens by a gel-forming substance which functions to block the opening in the lens capsule, preventing its escape. This treatment is preferably carried out in conjunction with laser induced phacofracture.
The U.S. Pat. No. 5,095,030 issued to Levy, et al discloses a group of hydro-monobenzoporphyrins xe2x80x9cgreen porphyrinsxe2x80x9d (Gp) having absorption maxima in the range of 670-780 nanometers, which are useful in treating disorders or conditions which are subject to hematoporphyrin derivative (HPD) treatment in the presence of light, or in treating virus, cells and tissues generally to destroy unwanted targets. The use of the Gp permits the irradiation to use wavelengths other than those absorbed by blood. The Gp may also be conjugated to ligands specific for receptor or to specific immunoglobulins or fragments thereof to target specific tissues or cells for the radiation treatment. Use of these materials permits lower levels of drug to be used, thus preventing side reactions which might destroy normal tissues.
The U.S. Pat. No. 5,957,914 issued to Cook, et al discloses a laser photo-optic breakdown probe and handpiece system especially suited for small-incision cataract removal including an optical fiber through which a pulsed source of laser energy is delivered to a target spaced from the distal end of the fiber. The fiber is routed through a first guide tube with the target mounted at an angle on the end thereof. Holes are provided in the guide tube for the flow of infusion fluid therethrough. A second aspiration tube surrounds and provides rigidity to the guide tube, shields the distal end of the fiber and target, and serves as a channel for aspirated fluids and dislodged cataractous material. The guide tube is affixed to a transfer housing to which infusion and aspiration connections are made. The aspiration tube forms part of a handle assembly which is detachably coupled to the transfer housing.
For background purposes and as indicative of the art to which the invention relates, reference may be made to the following remaining patents found in the search:
Non-Patent Documents:
Borkman, R. F. et al. xe2x80x9cThe Rates of Photodestruction of Tryptophan Residues in Human and Bovine Ocular Lens Proteinsxe2x80x9d, Exp Eye Res., 32:747-754, (1981).
Borkman, R. F. et al. xe2x80x9cFluorescence Lifetimes of Chromophores in Intact Human Lenses and Lens Proteinsxe2x80x9d Exp Eye Res., 32:313-322, (1980).
Overberger, C. G., et al., xe2x80x9cThe Preparation and Polymerization of rho-Alkylstyrenes. Effect of Structure on the Transition Temperatures of the Polymers,xe2x80x9d J. Am Chem. Soc., 75:3326-3330 (1953).
In the present invention, cataractous lens tissue, is liquefied by photoactive chemicals injected into the lens substance. Radiation is applied to specific regions of interest within the lens substance. The zone of lens tissue liquefaction is limited to an area which generally corresponds in size and diameter to applied laser beam radiation. Transparent fluid injected between the lens substance and capsule (hydrodissection) can protect the lens capsule from photolytic injury. As liquefaction proceeds the lens is microfractured, thus enhancing penetration of photolytic chemical available for further liquefaction. Liquefied lens tissue results in an emulsion of fluid and small fragments which may be aspirated through a tube having a diameter of about 750 microns or less. Irrigating fluid introduced within the lens capsule serves to maintain the intracapsular volume, prevent collapse of the capsular balloon,and to wash out residual cataractous lens material.
To accomplish liquefaction of lens tissue, a predetermined aliquot of photoreactive chemical is injected into the lens tissue. The chemical is administered by means of a syringe with needle that penetrates the lens capsule and engages lens tissue within the capsular balloon. Air or polymeric material delivered during the terminal phase of injection seals the lens capsule and prevents escape of phacolytic agents.
Irradiation of lens tissue containing the photosensitizing agent can be achieved by delivering light energy from an external source. The cataractous tissue is irradiated at the wavelength absorbed by the selected photoreactive agent. The present invention, therefore, represents a unique method of cataract removal by indirect photoablation.
A dual cannula surgical instrument of the present invention is employed to perform the above-discussed cataract removal. The instrument comprises a dual needle which delivers pliotoreactive chemicals into cataractous lens tissue and also removes degraded lens tissue remnants. The instrument additionally provides a conduit for the introduction of enzymes to aid in the penetration of photoreactive chemicals within the lens tissue. Aspiration of photolyzed lens tissue is accompanied by irrigation to sufficiently replace evacuated tissue. Aspiration in conjunction with irrigation serves to maintain the shape and integrity of the lens capsule during the surgical procedure.
Radiation is achieved through a light delivery means contained within the present invention The central needle serves as a guide for a fiberoptic which transmits laser radiation to lens tissue in order to perform photodynamic treatment. The fiberoptic in the central needle delivers radiation from an external point source into the capsular balloon. It has been found that optimum time intervals between administration of photosensitizing agent, illumination, and withdrawal of liquefied tissue, is dependent on adequate progression of liquefaction and judgment of the practitioner. The preferred length of time of irradiation and wavelength of light is also determined by the type and amount of photosensitve composition being used. The level of irradiance administered varies between 150-900 mW/cm2. Higher irradiances have the advantage of shortening duration of treatment. The interval of time from administration of the photosensitizing agent to light treatment can vary, depending on the density of the cataract tissue and penetration of photosensitizing agent. In general, it ranges from about one to twenty-five minutes.
Absorption of light energy by the photosensitive composition causes a reaction which destroys the tissue in which the composition has accumulated. The present invention, therefore, represents a new method and apparatus for cataract removal by photodynamic therapy. Advantageously, the present apparatus and method (referred to as phacophotolysis) permits minimally invasive cataract removal. The incision size required by phacophotolysis allows for a substantially smaller incision size than that required for conventional procedures and, importantly, without the deleterious effects of energy shock waves employed by ultrasonic or laser phacoemulsification. Complications associated with ultrasonic or laser phacoemulsification include heat injury to ocular tissue, damage to corneal endothelium, and rupture of the posterior capsule.
Following removal of decomposed cataractous tissue, a polymer matrix selected from a group consist of acrylate, methacrylate, silicone, styrene, thermopolymer, plastic polymer, photosensitive resin, organopolysiloxane, or organopolymer is then introduced into the body of the lens capsule to reconstitute the intralenticular space and serve as a refracting niedium. Additionally, an ultraviolet absorbing compound as discussed in U.S. Pat. No. 4,304,895 and U.S. Pat. No. 4,528,311 may be incorporated in the polymer matrix.
Modification of lens power by application of external laser energy is performed on lens replacement compositions that include photocurable polymers. The preferred photocurable polymers are derived from cyclic organohydrogenpolysiloxanes. Grubbs, et al (U.S. Pat. No. 4,919,151) teach that the time of curing of related synthetic polymers is about one minute at body temperature. Synthetic lens material of the type described herein are non-toxic and cures at body temperature of about 37 degrees Celsius. Polymerization and curing of the synthetic materials described herein are initiated by light using a photoinitiator such as dimethoxyphenylacetophenone,and further enhanced in the presence of an acid catalyst.
When using a biocompatible, non-immunogenic thermopolymer in a flowable, heated state, the transferred material is permitted to cool and conform to the internal shape of the capsular balloon. An intact capsule containing a flexible material restores the potential for lenticular accommodation and thus, the capability for focusing at various focal distances.
The substitute lens material can also be transferred into an expandable intralenticular balloon constructed of a resilient material selected from a group consisting of hydroxymethylmethacrylate, polymethylmethacrylate, polyethelene, polystyrene, polypropylene, polytetrafluorethylene, polybutylene terephthalate, polyether glycol, ethyl-vinyl-acetate, or it may be constructed of silicone. When inflated with substitute lens material, the inflatable balloon expands to conform to the intralenticular space. Detachable balloons are well known in the art and more fully described in U.S. Pat. No. 4,395,806 (Wonder et al). After the method has been completed, the delivery instrument is removed from the eye. The polymer material remaining within the lens capsule or intralenticular balloon is treated by an external source of radiation, such as an ultraviolet laser, to alter the refractive state of the eye.
These and other novel advantages of the present invention will become apparent from the subsequent detailed description of the preferred embodiment and the appended claims taken in conjunction with the accompanying drawings.